1. Field of the Invention
The present invention relates to a composite material and a method for producing the same, the composite material being used for heat sinks for semiconductor devices, for constructing a heat sink for a semiconductor device for efficiently releasing heat generated from the semiconductor device.
2. Description of the Related Art
In general, heat is a dangerous enemy of semiconductor devices. Therefore, it is necessary that the internal temperature of the semiconductor device does not exceed a maximum allowable temperature for retaining the joining or connecting structure. Semiconductor devices such as power transistors and semiconductor rectifying elements consume a large amount of electric power per unit of operation area. Therefore, it is impossible to release a sufficient amount of the generated heat only by relying on an amount of heat released through a case (package) and lead wires of the semiconductor device. In such a circumstance, there is a fear that the internal temperature of the device is raised, and thermal destruction would occur.
This phenomenon also occurs in the same manner in semiconductor devices which carry a CPU. The amount of heat generation during operation is increased in proportion to the improvement in clock frequency. As a result, it is an important factor to make a thermal design in consideration of heat release.
In the thermal design in consideration of avoidance of the thermal destruction or the like, element designs and mounting designs are made taking account of a heat sink having a large heat release area which is securely attached to a case (package) of the semiconductor device.
In general, those used as the material for the heat sink include metal materials such as copper and aluminum having good thermal conductivity.
Recently, in the semiconductor devices such as CPUs and memories, it is intended to drive the device with low electric power in order to decrease electric power consumption, while the semiconductor device itself tends to have a large size in proportion to highly densified element integration and enlargement of element formation area. When the semiconductor device has a large size, the stress, which is generated due to the difference in thermal expansion between the semiconductor substrate (silicon substrate or GaAs substrate) and the heat sink, is increased. As a result, there is a likelihood of delamination and mechanical destruction of the semiconductor device.
In order to avoid such inconveniences, the conceivable countermeasure includes realization of low electric power operation of the semiconductor device and improvement of the material for heat sinks. At present, in relation to the low electric power operation of the semiconductor device, a device, which is operated at a power source voltage of a level of not more than 3.3 V, is practically used, beyond those operated at the TTL level (5 V) having been hitherto used.
On the other hand, in relation to the constitutive material for the heat sink, it is insufficient to consider only the thermal conductivity. Besides, it is necessary to select a material having high thermal conductivity with a coefficient of thermal expansion which is approximately coincident with those of silicon and GaAs to be used for the semiconductor substrate.
A variety of reports have been submitted in relation to the improvement in material for the heat sink. For example, there is a case based on the use of aluminum nitride (AlN) and a case based on the use of Cu (copper)-W (tungsten). AlN is excellent in balance between the thermal conductivity and the thermal expansion, and it especially has a coefficient of thermal expansion which is approximately coincident with that of Si. Therefore, AlN is preferred as a material for heat sinks for the semiconductor device based on the use of a silicon substrate as the semiconductor substrate.
On the other hand, Cu-W is a composite material which possesses both the low thermal expansion of W and the high heat conductivity of Cu, and it easily processed by means of machining. Therefore, Cu-W is preferred as a constitutive material for heat sinks having complicated shapes.
There are other suggested cases including, for example, a material obtained by containing metallic Cu in a ratio of 20 to 40% by volume in a ceramic base material comprising a major component of SiC (Conventional Example 1: see Japanese Laid-Open Patent Publication No. 8-279569), and a material obtained by impregnating a powdery sintered porous body comprising inorganic substances with Cu in an amount of 5 to 30% by weight (Conventional Example 2: see Japanese Laid-Open Patent Publication No. 59-228742).
The material for heat sinks concerning Conventional Example 1 is based on powder shaping in which a green compact comprising SiC and metallic Cu is shaped to prepare a heat sink. Therefore, the coefficient of thermal expansion and the coefficient of thermal conductivity thereof are persistently represented by theoretical values. In this case, there is a problem that it is impossible to obtain the balance between the coefficient of thermal expansion and the coefficient of thermal conductivity demanded for actual electronic parts or the like.
In Conventional Example 2, the ratio of Cu, with which the powdery sintered porous body comprising inorganic substances is impregnated, is low. Therefore, there is a fear that a limit appears when it is intended to increase the thermal conductivity.